Method for improving the reduction degree in the smelting of ferroalloy

ABSTRACT

The invention relates to a method for improving the reduction degree of metal components in a chromite concentrate when smelting ferroalloy suitable for manufacturing of stainless steel. The chromite concentrate is fed together with nickel-containing raw material so that by means the amount of nickel-containing raw material it is achieved a desired reduction degree for the metal components of ferroalloy.

This invention relates to a method for improving the reduction degree ofmetal components in a material to be treated when smelting ferroalloy,as ferrochrome suitable for manufacturing of stainless steel. Accordingto the method nickel-bearing material is fed into ferroalloy.

It is known from the WO patent publication 2010/092234 a method whereinnickel ore and/or nickel concentrate or an intermediate productprecipitated from solutions of nickel ore and/or nickel concentrate isagglomerated in the manufacturing process of ferrochrome so that it isfirst produced from nickel-containing material together withiron-containing chromite concentrate and binder agent pellets, and thedrying and calcination of nickel-containing material is carried outadvantageously within one-stage heat treatment of pellets, sintering.With the heat treatment of pellets the object are strengthened so thatthe heat treated objects are conveyable, when desired, essentiallycomplete between separate process stages. If needed, the pellets can bepreheated before sintering. Heat treated objects can be conveyed, whendesired, essentially complete between separate process stages. Heattreated objects can be downsized, when desired, when conveying objectbetween separate process stages or process units. Sintered and thusstrengthened pellets are used as material in a smelting process carriedout in reducing conditions, in which case it is received as a smeltingproduct nickel-containing ferroalloy, ferrochromenickel.

The above mentioned WO patent publication 2010/092234 thus relatesmainly to the production of nickel-containing pellets by sintering.Instead, smelting conditions of the sintered pellets are not exactlydescribed. When describing the energy efficiency it is, however,mentioned that nickel containing in pellets catalyses chromium reductionin pellets and thus decreases the specific consumption, advantageouscarbon, of the reducing agent in the ferroalloy production.

It is now surprisingly observed that nickel containing in pellets notonly catalyses the reduction of chromium in chromite pellets, but nickelcontaining in the feed of a furnace used for smelting of chromiteimproves in the smelting process the reduction of all essential metalcomponents, iron, chromium and nickel, containing in the feed of thesmelting furnace. The object of the present invention is to utilize thissurprising finding and to achieve a more effective method than beforefor increasing the reduction degree in the smelting process of chromitematerial in which method the reduction of metal components in chromiteduring the smelting is improved by alloying into the material to be goneinto smelting nickel-containing material and simultaneously to achieve aprealloy, ferrochromenickel, suitable to the production of stainlesssteel. The essential features are enlisted in the appended claims.

According to the invention, it is alloyed into the raw material, aschromite, to be smelted in the ferroalloy production before the smeltingnickel-containing material, in which case nickel-containing improves thereduction of metal components containing in the feed materialsimultaneously when nickel-containing material itself is managed to bereduced as a metallic component in the ferroalloy. According to theinvention, by means of the nickel amount to be added into the ferroalloyit can advantageously be adjusted the reduction degree of metalcomponents in the ferroalloy and simultaneously be achieved a ferroalloycontaining the desired nickel content, as ferrochromenickel alloyshaving different nickel contents. Ferrochromenickel alloys containingdesired nickel contents can be used for instance for the production ofdifferent stainless steels, as austenitic or duplex stainless steels.

In the method according to the invention it can be used as anickel-containing raw material at least partly nickel oxide, at leastpartly nickel ore and/or nickel concentrate or at least partly anickel-containing intermediate product achieved by the leaching and/orby precipitating of nickel ores and/or nickel concentrates. Thenickel-containing raw material is fed into a smelting process togetherwith ferrochrome raw material. Before feeding into a smelting furnacethe nickel-containing raw material is pretreated either so that sinteredpellets are formed from the nickel-containing material together with theferrochrome raw material or so that the nickel-containing raw materialis pretreated separately to chromite pellets. It is possible to carryout the pretreatment of the nickel-containing raw material also so thatone part of the nickel-containing raw material to be fed into thesmelting furnace is pretreated together with chromite pellets and onepart of the nickel-containing raw material is pretreated separately tochromite pellets. Thanks to different pretreatments thenickel-containing raw material to be fed into the smelting furnace andpromoting the reduction of different metal components can be forinstance partly nickel-containing hydroxide intermediate product, partlysulphidic or lateritic nickel concentrate.

The nickel-containing raw material to be utilized in the methodaccording to the invention is advantageously a nickel-containinghydroxide intermediate product from mines or other hydrometallurgicalprocesses, which intermediate product is precipitated from solutions oflateritic and/or sulphidic nickel ores and/or nickel-containingconcentrates of sulphidic ores. This kind of nickel-containing hydroxideintermediate product is for instance a nickel-containing intermediateproduct from pressure leaching, atmosphere leaching or heap leaching oflateritic or sulphidic nickel ores or nickel concentrates as well as anickel-containing precipitated product of solvent extraction solutions,stripping solutions or refining solutions received from solventextraction processes or ion exchange processes of nickel-containingmaterials. In the method of the invention it can as a raw material beused also carbonate or sulphate nickel materials. Further, a sulphidicnickel concentrate itself and a hydrometallurgically precipitated nickelsulphide intermediate product are suited for the nickel-containing rawmaterial of the method.

According to the invention, the amount of the nickel-containing materialto be fed into a smelting furnace is adjusted in the range of 5-25weight %, preferably 10-20 weight %, from the total mass of thepretreated material to be fed into the smelting furnace. When adjustingthe amount of the nickel-containing to be fed into the smelting furnaceit is considered the achievement of the energy-economically favourablereduction conditions and/or the production of a prealloy,ferrochromenickel, suitable the production of favourable stainless steelin each case. Using a small addition of nickel-containing raw material,the reduction degree remains low, in which case it is created aferroalloy with low nickel content, ferrochromenickel. This kind offerroalloy with a low nickel content is a favourable prealloy especiallyto the production of duplex stainless steel grades. Using a greateraddition of nickel-containing raw material the reduction degreeincreases and also the nickel content in the smelting product isgreater. This kind of ferrochromenickel with a greater nickel content isfavourable to use to the production of austenitic stainless steel gradeshaving a high nickel content.

In the pretreatment of nickel-containing raw material to be fed into asmelting furnace in accordance with the method of the invention it isadvantageously considered the composition and the microstructure of thenickel raw material. If the nickel-containing raw material is forinstance a nickel-containing intermediate product of mines or otherhydrometallurgical processes precipitated from solutions ofnickel-containing solutions, which intermediate product requires tocarry out as a pretreatment among others calcination at a highertemperature, the pretreatment of the nickel-containing raw material iscarried out together with the production of chromite pellets andsintering of pellets. Instead, if the nickel-containing raw material ofthe method according to the invention is material, as for instancenickel oxide, nickel ore and/or nickel concentrate, which does notrequire in addition to a possible drying any other essentialpretreatment at a higher temperature, then the nickel-containing rawmaterial is possible to feed into a smelting furnace with the feeding ofchromite pellets. The microstructure and composition of thenickel-containing raw material can also be such that it is advantageousto pretreat the raw material separately from chromite pelletizing and tofeed the nickel-containing raw material into sintering of chromitepellets before feeding into a smelting furnace.

In the method according to the invention it is used advantageously asmelting furnace which is provided with a preheating equipment so thatthe feed going into the smelting furnace is conducted through thepreheating equipment into the smelting furnace. According to theinvention the pretreated nickel-containing raw material is conductedalso into the preheating equipment wherein the nickel-containing willcome at the latest into contact with other material to be fed into thesmelting furnace. In the smelting furnace the nickel-containing togetherwith chromite pellets are smelted to ferrochromenickel having a desiredcomposition, which ferrochromenickel can be utilized in accordance withits composition advantageously for instance in the production ofaustenitic or duplex stainless steels.

When according to the invention smelting of the nickel-containing rawmaterial is carried out advantageously in a closed submerged arcfurnace, carbon monoxide gases generated in the reduction and smeltingcan be utilized in one hand for instance in the sintering of chromitepellets and in possible other pretreatment and preheating, in anotherhand for instance in different steps of the production path of stainlesssteel produced from the smelting product, ferrochromenickel.

The method according to the invention is described in more details bymeans of the appended example.

EXAMPLE

From a chromite concentrate containing iron and chromium and anintermediate product containing nickel it was formed a mixture, intowhich mixture it was added as a binder 1,2 weight % bentonite and 3weight % slag forming material, flux, either limestone or wollastonite.In the table 1 it is presented the contents of chromium, iron, nickel,carbon and sulphur as weight % in mixtures, into which was added 10weight % (Test 1) and 20 weight % (Test 2) nickel hydroxide. Further, inthe table 1 it has as a reference material (REF) a mixture, into whichmixture nickel hydroxide was not added.

TABLE 1 Cr Fe S weight % weight % Ni weight % C weight % weight % REF28.3 18.3 0.3 0.12 0.06 Test 1 26.5 16.8 5.3 0.10 0.03 Test 2 24.4 15.110.1 0.10 0.03

The mixtures containing a binder and representing each material of thetable 1 were pelletized and sintered. A part of sintered pellets was fedrepresentatively into a smelting furnace with a slag former and areducing agent.

The materials according to the table 1 were smelted, and in the table 2it is presented the contents of chromium, iron, nickel, carbon andsilicon in smelting products in question and further the recovery of themetal components, chromium, iron and nickel, into the smelting product.The carbon content is composed in accordance with the composition andthe equilibrium of the metal alloy. The feed batch has carbon so muchthat carbon is some enough also for the reduction of silicon into thesmelting product. The feed alloy has silicon oxide in raw material andin production bulk supplies.

TABLE 2 Contents (weight %) Recoveries Cr % Fe % Ni % C % Si % Cr % Fe %Ni % REF 53.5 33.4 0.36 8.1 2.4 88.9 90.3 — Test 1 49.8 30.1 7.1 6.7 2.886.6 88.7 86.0 Test 2 46.2 26.9 13.3 6.1 4.2 91.5 90.1 88.6

For one part of sintered pellets it was made in the laboratory scalethermogravimetric measurements in order to monitor the reduction degreeof the metal components, chromium, iron and nickel, of pellets in theconditions representing the smelting process at different temperaturezones with the maximum temperature of 1550° C. In the table 3 it ispresented the results of the thermogravimetric measurements for thereduction degree of chromium (Cr_(met)/Cr_(tot)), iron(Fe_(met)/Fe_(tot)) and nickel (Ni_(met)/Ni_(tot)) at the temperaturesof 1400° C. and 1550° C.

TABLE 3 (Cr_(met)/Cr_(tot)) % (Fe_(met)/Fe_(tot)) % (Ni_(met)/Ni_(tot))% REF (1400° C.) 1.1 16.8 — REF (1550° C.) 6.1 47.2 — Test 1 (1400° C.)2.6 37.4 67.3 Test 1 (1550° C.) 15.4 70.6 78.9 Test 2 (1400° C.) 5.256.7 79.1 Test 2 (1550° C.) 57.4 94.3 99.1

The addition of the nickel-containing raw material into pelletsincreases the reduction degree of chromium and iron at the temperatureof 1550° C. substantially, chromium more than 15% and iron more than 70%simultaneously when the reduction degree of nickel increases near to100% with the Test 2 nickel content. The increase of the reductiondegree for all metal components, chromium, iron and nickel in sinteredpellets by means of the addition of a nickel-containing raw materialsimultaneously decreases the need of coke used as reducing agent in theachievement of the reduction conditions of the smelting process.

1. Method for improving the reduction degree of metal components in achromite concentrate when smelting ferroalloy suitable for manufacturingof stainless steel, characterized in that the chromite concentrate isfed together with nickel-containing raw material so that by means theamount of nickel-containing raw material it is achieved a desiredreduction degree for the metal components of ferroalloy.
 2. Methodaccording to the claim 1, characterized in that the nickel-containingraw material is fed 5-25 weight %, advantageously 10-20 weight % of thetotal amount of the material to be fed into the smelting furnace. 3.Method according to claim 1, characterized in that during the smeltingit is reduced at least 2.6% of chromium containing in the chromiteconcentrate.
 4. Method according to claim 1, characterized in thatduring the smelting it is reduced at least 37.4% of iron containing inthe chromite concentrate.
 5. Method according to claim 1, characterizedin that at least one part of the nickel-containing raw material is fedinto the smelting furnace within pellets produced from the chromiteconcentrate.
 6. Method according to claim 1, characterized in that atleast one part of the nickel-containing raw material is pretreatedseparately from the chrome concentrate pellets before feeding into thesmelting furnace.
 7. Method according to claim 1, characterized in thatit is fed into the smelting furnace as the nickel-containing rawmaterial at least partly nickel oxide.
 8. Method according to claim 1,characterized in that it is fed into the smelting furnace as thenickel-containing raw material at least partly nickel ore and/or nickelconcentrate.
 9. Method according to claim 1, characterized in that it isfed into the smelting furnace as the nickel-containing raw material atleast partly a nickel-containing intermediate product achieved by theleaching and/or by precipitating of nickel ores and/or nickelconcentrates.
 10. Method according to the claim 9, characterized in thatit is fed into the smelting furnace at least partly nickel-containingintermediate product achieved by pressure leaching of lateritic orsulphidic nickel ores or nickel concentrates.
 11. Method according tothe claim 9, characterized in that it is fed into the smelting furnaceat least partly nickel-containing intermediate product achieved byatmospheric leaching of lateritic or sulphidic nickel ores or nickelconcentrates.
 12. Method according to the claim 9, characterized in thatit is fed into the smelting furnace at least partly nickel-containingintermediate product achieved by heap leaching of lateritic or sulphidicnickel ores or nickel concentrates.
 13. Method according to the claim 9,characterized in that it is fed into the smelting furnace at leastpartly nickel-containing precipitated product of nickel-containingsolvent extraction solutions.
 14. Method according to the claim 9,characterized in that it is fed into the smelting furnace at leastpartly nickel-containing precipitated product of nickel-containingstripping solutions.
 15. Method according to the claim 9, characterizedin that it is fed into the smelting furnace at least partlynickel-containing precipitated product of nickel-containing refiningsolutions.
 16. Method according to claim 1, characterized in that it isfed into the smelting furnace as nickel-containing material partlynickel concentrate, partly a nickel-containing intermediate productachieved by the leaching and/or by precipitating of nickel ores and/ornickel concentrates.